In known plunger stroke apparatuses, a gas cushion is moved by moving a plunger in a cylinder, in order to aspirate a liquid sample into a pipette point or another sample pickup portion or to eject it from it. The volume is determined with the aid of the volume displacement of the gas cushion, which is proportional to the adjusted path of the plunger. As an adjustment member or actor unit, respectively, for moving the plunger in the cylinder, a stepping motor is used for instance, which accurately hits working points for taking up and releasing liquid, in a fashion that is independent of the working load. After reaching the working points, it is waited until the taking or releasing of the liquid is complete. The dosing is based on the fact that pressure compensation up to reaching a static balance takes place between gas cushion, liquid in the pipette point and surroundings.
From EP-A-1 150 105 a gas cushion microdosing system for dosing liquids in the microliter and sub-microliter range is known. The system has a liquid reservoir with a storage room for the liquid that is to be dosed, the boundary of which is perforated with a liquid passage guiding to the outside and a gas passage. A gas displacement system with a micro pump for pumping a gas has a connection to the gas passage. A dosing control unit for generating a negative pressure or an overpressure by actuating the micro pump and for pressurising the liquid reservoir with the negative pressure or overpressure is in interacting connection with the micro pump, in order to take up liquid through the liquid passage into the storage room or to release it from the same. The system has a pressure sensor for detecting the pressure in the gas displacement system, with which the dosing control unit is in interacting connection in order to determine the taken or released liquid volume or to regulate it towards a predetermined value. The dosing control unit controls the negative pressure or the overpressure in the gas displacement system when aspirating or releasing liquid by detecting the pressure existing therein by means of the pressure sensor and regulating the pumping rate or the volume displacement of the micro pump towards a predetermined value or course of that value, and determines the taken up or released liquid volume or regulates is towards a predetermined value, via the pumping rate or the volume displacement of the micro pump.
The determination of the released volume by knowing the pumping rate is time-critical and imprecise, in particular when smaller liquid volumes are to be released at relatively high overpressures in a free stream and the pumping rate is pressure-dependent. Even taking gas equations of state as an aid is also imprecise, by reason of the fast volume displacement for generating the overpressures, that has to be known, and thus is not suited for the determination of liquid volumes that are to be released.
Departing from this, the present invention is based on the objective to provide a method for dosing liquid volumes which enables accurate dynamic detection of the liquid volume that has been dosed. Further, the present invention is based on the objective to provide an apparatus for dosing liquid volumes, suited for practising the method.